rename plasm and fix links

[Maar-io]

Pull Request Summary

Rename plasm to Astar and fix links

Check list

• [ ] contains breaking changes
• [ ] adds new feature
• [ ] modifies existing feature (bug fix or improvements)
• [ ] relies on other tasks
• [x] documentation changes
• [ ] tests and/or benchmarks are included
• [ ] changed API client type definition or chain metadata

This pull request makes the following changes:

Adds

• (ex: Add feature A)

Fixes

• (ex: Fix validation function)

Changes

• (ex: Change document B)

To-dos

*Feel free to remove this section if it's not applicable

• [ ] (ex: add user list)

[hoonsubin]

When can we merge this?

[akru]

@Maar-io could you rebase it?

[akru]

Probably it's outdated, let me close it